The present invention generally relates to gas lubricated bearings associated with the rotating shafts of gas turbines, turbochargers, and turbocompressors, and more specifically, to an apparatus and method for cooling such bearings by dissipating heat from the bearings.
In gas turbine engines, or any other machinery in which high temperature rotors are situated adjacent to high speed bearings, it is essential to maintain the bearings at an acceptably low temperature. This is normally achieved, when the engine is running, by providing the bearings with an adequate supply of lubricating oil from a pump driven by the engine or an external motor driven pump. However, this supply of cooling oil ceases when the engine stops, and the residual heat in the turbine rotor can be sufficient to damage the bearings adjacent the turbine rotor. Such bearings are especially vulnerable to the effect of heat-soak in the case of a gas turbine engine having a regenerative heat-exchanger and a thermally insulated main casing, because the residual heat in the turbine rotor on shutdown is virtually prevented from being dissipated to the atmosphere.
Bearings located adjacent to turbine rotors are exposed to temperatures which are sometimes higher than is desirable because of conduction and the radiation of heat from the elevated temperature rotor. This may occur during steady state operation as well as after shutdown due to transfer of heat from the hot turbine wheels through the thermally conducting shaft to the bearings. In either case, the temperature rise occurring in the bearings may be more than can be tolerated by either the materials of which the bearing or the lubricant is constructed.
It is important that the material limits and restrictions involving the bearings be maintained in order to provide the desired service life of the machinery. If the temperature exceeds acceptable limits, there may be a loss of wearability of foil bearing coatings or, in the extreme case, evaporation and destruction of the coating. Similarly, the foil material may creep if the temperature is too high, with attendant loss of elasticity and load bearing capacity.
The prior art appears to disclose at least one attempt to use bleed air taken from the compressor to cool particular components in turbomachinery, such as turbine blades, turbine exhaust, bearings and/or interstitial spaces within the machinery. In U.S. Pat. No. 5,087,176 to Wieland (“Wieland patent”), an apparatus for cooling pliant film bearings incorporates a shaft. Cooling air is circulated through the interior of the shaft and the bearing shaft is isolated from a turbine wheel. Bleed air may be directed from the compressor to cool the foil bearings. However, the Wieland patent does not disclose any criteria for assigning a location for the point in the compressor where bleed air may be drawn from the compressor. Additionally, the Wieland patent does not disclose any relationship between the size of the bleed hole through which the air is bled and the size of other chambers within the engine.
As can be seen, there is a need for an improved apparatus and method for cooling bearings that take advantage of useful bleed point locations, or bleed hole and chamber size relationships.